Conventional methods for softening hard water commonly utilize lime (calcium hydroxide) as a chemical treatment additive to precipitate scale-forming ions from the hard water. The lime serves as a hydroxide ion donor, which induces the formation of insoluble precipitates, such as calcium carbonate and magnesium hydroxide. The resulting precipitates are discharged from the water to obtain a soft water product. Soda ash (sodium carbonate), may also optionally be employed with the lime to facilitate the formation of insoluble precipitates. The soda ash serves as a bicarbonate ion donor in the event there are insufficient bicarbonate ions in the feed water for effective operation of the lime. Examples of lime/soda ash treatment processes are disclosed in U.S. Pat. Nos. 3,951,806 and 3,929,640.
It has been found that effective utilization of lime in hard water treatment processes requires strict control of the amount of lime added to the hard water for optimum hardness reduction. If insufficient lime is added, the hardness of the water is inadequately reduced. If excess lime is added, the hardness of the treated water can increase because the excess lime serves as a calcium ion donor to the treated water. Additionally, introduction of excess lime into the hard water can generate an inordinate quantity of precipitates, causing a disposal problem for the resulting sludge waste product. FIG. 1 graphically demonstrates an optimal range of the amount of lime which is added to the hard water to optimize hardness reduction in the treated water. The optimal range resides between the vertical dashed lines of the graph.
Although the need to strictly control lime addition in hard water treatment processes for optimum hardness reduction is generally recognized, process optimization has been difficult to achieve in practice, primarily due to the relatively narrow optimal operating window for lime addition. U.S. Pat. No. 5,340,468 exemplifies recognition of the need to control lime addition to optimize certain hard water treatment processes and teaches a lime addition control system to achieve this objective. However, such systems are often not practical because of relatively complex operational requirements and relatively high operating and maintenance costs.
Another conventional method for softening hard water substitutes caustic (sodium hydroxide) for lime as an alternate hydroxide ion donor. U.S. Pat. No. 4,969,520 teaches such a hard water treatment process utilizing caustic. Caustic is a generally more effective treatment additive than lime because caustic does not contain calcium ions. Caustic obviates the risk of introducing additional hardness into the treated water due to the presence of excess treatment additive. As demonstrated by FIG. 2, the hardness of the treated water does not increase when the amount of caustic added to the water exceeds the amount required to achieve maximum hardness reduction depicted by the dashed vertical line. Nevertheless, U.S. Pat. No. 4,969,520 teaches that hard water treatment processes utilizing caustic have practical limitations because it is generally not cost effective to employ caustic for the reduction of hardness in water below predetermined minimum levels. Unfortunately, these predetermined minimum hardness levels may exceed the requirements of the user. As such, the present invention recognizes a need for a more cost effective process of treating water to reduce its hardness. Accordingly, it is an object of the present invention to provide an effective process for treating a hard water to reduce the hardness thereof. More particularly, it is an object of the present invention to provide an effective process for reducing the hardness of an oilfield produced water. It is another object of the present invention to provide a process for reducing the hardness of an oilfield produced water which has relatively low chemical costs. It is yet another object of the present invention to provide a process for reducing the hardness of an oilfield produced water that reduces the amount of sludge that is produced as waste. These objects and others are achieved in accordance with the invention described hereafter.